A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,617
Sometimes you can use the ticket number to pull up your reservation in another airline's system and then note the PNR and select seats.
However, I think it's a good idea regardless to always call the other carrier and check that they can see your reservation AND a matching ticket. Be sure to note their PNR. You can select seats, etc. during the phone call.