There's no way to do it online or offline - for more than one person to be on the same PNR, the booking classes have to match (c.f. when you try and book multiple seats in a class which works out at the price you expect for, say, one person but not for two people - if the buckets were J9C9D9I1, for example, this is due to the system automatically booking you both in to D instead of one in I and one in D).

Unless you mean one segment in CE and one in ET, in which case, yes...price up a booking in ET and upgrade whichever leg you want on the final screen.