I am in a position close to yours. I have been a CO flyer forever (NJ will do that to you) and have always had a second program. A few years ago, I went with UA for my second program, got comp'ed to PremEx and have enjoyed having the pair. I am able to fly enough to keep Plat on CO and 1K on United, and that has served me well.

One thing I have noticed is that I have a lot more choices for award tickets by having miles in two programs. A couple of times I was shut out of my preferred dates on SkyTeam, and *A came through.

I also have had problems with mileage accrual on SkyTeam, due to some of CO's rules (especially with lower fares and excluded fare classes like Q on KE). I have loved UA, and especially the international upgrades. The combination has worked out very well for me.

The one area where our situations differ is your statement about your company buying J for long trips. To me, the biggest benefits of UA have been the E+ seats and the systemwide upgrades. It sounds like you may not have as much need of systemwides.

Personally, the added flexibility of having two programs works for me, and I encourage it often. However, if you think you can only make the highest level on one program, and CO has been working for you (and you normally buy fares that qualify for EQM), then I would suggest sticking with that.