Welcome to FT!

Well, a couple years ago there were ways to do those two airlines via Amtrack but that portal ended and is no more. One never knows, however, if and when a new loophole or deal will come up outta nowhere, but things do seem to be more tied down than they once were across the board.

You cannot really "combine" miles with another airline especially ones from different "alliances" but you could maybe figure out what your balances are on both and go at it a different way.

First, let's say your balance on CO is 13,500 and your balance on UA is 18,000. You could learn--from forum threads throughout FT-- about special promos, credit cards, offers and online mall purchases, etc how to "bring up"* those balances to the std. award level on most airlines of 25k for a free domestic ticket. Now, once you have 25k on both, which could take a bit of time but could be worth it, you can try something like this:

Find some friend or family member or co-worker who is flying, say, on CO or a partner airline of them, and book them an award ticket using your 25k miles. Then, in return, maybe that same person has enough of his or her own United miles to book YOU a ticket on UA! AND, if you, say, have a partner you wish to fly with on UA, then you simply use your 25k UA miles to book the second ticket and viola! 2 people fly together on the same flight and it all started with two different airlines!

Beware, however, that outwardly and knowlingly bartering for miles or awards on any airline is strictly forbidden and could cost you all your miles should you ever get caught doing it! However, I seriously doubt anyone would blink an eye if they knew you were considering trying the above with your mom or brother or something! Just be smart and tactful in this stuff. ok?

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Now, to get all these miles and organize all this takes some savvy, time, patience and an almost addiction of hobby-like ferver.

You have just come to the right place for that!
If you can tell us things like where you may be from, where you plan to fly from, what cities or regions you fly to, and which airline may be more to your liking, then surely the many experienced and friendly people in this huge forum will gladly serve up tips and tricks on how to best maximize your flying potential on either carrier or on others.

Try to learn something more about the partnerships or alliances of these airlines too. (PS: Get to know some of the airline acronyms, codes and nic names too)

Right now, CO, DL, NWA are among the US-based airlines in SKY TEAM
UA, US are part of Star Alliance
AA, BA and AS are One World partners.

etc etc.

AS is also partners with NWA. Good to know for such efforts as your own.

*Ways to bring up balances on the fast track include credit cards and special timed promotions found in every airline. Learn about the credit card offers --not from the little ads you see in in-flight mags or those generally seen on TV, rather, seek out info in here on special offers, hidden promos and such.

Here's a site that many find useful and I am sure you will too. This bloke posts here regularly and his site is well maintained and choc full 'O tips and deals you need to know:

http://www.freefrequentflyermiles.com/index.htm

Have any questions while you ramp up to be a Uber-elite flyer extraordinare? well, first do a search in the forum when you need to find things, because there are threads in here on nearly EVERYTHING. Be sure, in any mile-earning endeavor, you keep and organize ALL things related to whatever you are trying to earn in your account. I keep boarding passes and itinerary info until I see all miles post, I keep that same info if I fly a partner airline and am waiting on miles to post on the account, I keep printouts and screen shots of web sites (not just links, because links can get shut down) from things I sign up for online or buy on the airline site mall portals (note: United's seems to be my least favorite because they often botch up the mileage postings and take too long to get it right) and I keep all receipts and notes on whatever I buy or do with customer service in phone calls, etc. Renting cars, buying dinner, hotels stays and the like among them.

This becomes almost like a little admin job on the side of your real life. It takes you over and your significant other rolls his or her eyes but then you both enjoy the benefits later on. You sign up for all these credit cards and worry about your credit rating only to find out that the best card is the SPG AMEX and unless you need a VISA/MC for some things, you resort to going solely with that card for the best mileage and point results. You get into hotel points too, and then you die and no, all your miles don't go with you.

Once again, welcome to FT!

MM