A search came up empty - Is it possible to mix fare classes when booking? I want to book an H from EWR to FRA, and then a standard coach fare from BRU back to EWR (so I can u/g only on the outbound).

It is possible to do this online or do I have to call?

Thanks.

I think the answer is "yes", if the fares are available.

A sort of different question is: Can you book 2 pax on same itinerary, but different fare classes, on same confirm#? For example, one on U and the other on H for same flight.

It is possible, but not online. When looking at booking, make sure that you're getting the value for the H fare. If the upgrade isn't available to confirm immediately then you may end up throwing away money for a couple thousand EQMs, probably not a good deal. Even if the upgrade is available to confirm immediately the H fare may be more than the discounted fare + copay.
Definitely not online. I suppose a phone agent could piece it together, but I'm not sure I'd spend the time/energy to do so. The advantages are very, very minimal and it seems like it'd be a lot of effort to go through.

I booked ATL-EWR-SJO-EWR-ATL in I,W,I,W fare classes, so it is possible.

If your dates are flex, I have always been able to get confirm upgrade to FRA on flights that leave Saturday. In fact I am going this sat and no problem upgrades on the outbound and return. Other days of the week are hit and miss, but always seem to be lucky on sat.

On purpose or just beause that's how it priced out? I don't believe you can force segment by segment fare classes, though you can specify class of service on the different segments.

EWR-FRA is operated by 777 on saturdays which help out when looking to upgrade .

I agree with sbm, I've done it by calling Continental.

This is very good advice. I'd like to build on this, and add to it. What you should really do is look at all the fare classes between the lowest available and the H fare, as well as the copay schedule, and try to determine the sweet spot to get the best value.

In this case, I picked some random dates for EWR-FRA. While the H fare is slightly cheaper than the lowest fare available on the date (B fare) + copay, your best value is actually in the middle: an M fare. Below are are one-way base fares:

H fare = $826 + $0 copay = $826
B fare = $490 + $350 copay = $840 (lowest base fare for this date)
M fare = $565 + $200 copay = $765

So unless you need the refundability of the H fare, or you have an employer (or someone else) paying for your ticket, the lowest cost way to get into BF on this flight is the M fare + copay. But as sbm12 has pointed out, when buying up a fare class, you risk paying for a higher fare class and getting nothing, should your upgrade not be immediately confirmable.

What would I do with these data points?

Well, the M fare is only $75 more than the lowest in the market, so that's a risk I'd be willing to take. I would notbuy the H fare, as it's $336 more than the lowest fare, and if your upgrade does not clear, you have nothing to show for it other than a few more EQMs (which you may or may not need). Should the M fare clear you into BF, you will have gotten into BF for $61 cheaper than the lowest in the market.

The fare class calculation is somewhat incomplete without counting the EQMs if you're pursuing status. CO lists EWR-FRA as 3,869 EQMs. M earns 100% EQMs and H earns 150% EQMs (plus one additional EQP if you're qualifying on segments).

So for an H fare on this route you can earn an additional 1,934 EQMs for the extra $61 quoted. That translates into 3.2c/eqm.

It's then up to the OP to decide whether he's looking for more EQMs this year and whether this is a good value to him given the remainder of his travel patterns and opportunity cost.

Of course this all also presumes the availability of a BF mileage upgrade.

Build what you want on CO.com and then hold it by selecting, "Pay with cash." Then, call the web support desk at 800.300.1547 option 3. Ask the agent to twiddle the fare class of the flight you wish and to reprice the reservation for you. Then, purchase via CO.com. This gets you 100% EQM and the fare classes you wish. Travelers must all have the same fare class for each flight. If you want different ones you'll have to use additional PNRs.

It is possible online, but not under your control. If the cheapest tickets for different segments are in different fare buckets, you get different fare buckets. It happens all the time.

I'm pretty sure the answer is no. If the cheapest bucket has one seat left and you are buying two, both will be in the higher fare bucket.

To answer the original question (not whether it makes any sense to do it), the easiest way is to book the lowest fare class all the way through up to but not including purchase. At the last possible moment, hit the cancel button. You'll get a pop up window with your booking locater (held for 3 Hours, IIRC) and the telephone number to co.com support. Call, select the option for co.com support (3 the last time I called), tell them the booking locater and what you're trying to do. They can change the fare class of any segment (assuming it's available) and the whole thing remains a co.com purchase.

Why would you want to do this? If you just want to get two people on the same flight in different cabins I would think it would be easier to have two browsers open at the same time and book them simultaneously. That is how I book online when I am combining award flights for the kids and paying for my ticket. I would also consider that if paying for all of us as the boys don't have status so I would have to make a phone call to split the record to make sure I don't miss an upgrade.

Different fare classes does not equal different cabins. There are lots of coach fare classes. Why do this? Say you (elite) fly with the Mrs. or Mr. (non-elite). Same PNR makes it possible (w/o calling and begging) to put the Mr. or Mrs. in the elite section. You want H or Y for the increased EQMs, but the spouse doesn't care and gets a lower coach bucket.