Horizontal Bi-Amping question

I personally would use either the Adcom by itself or 4 of the 5 channels of the Marantz for bi-amping your speakers. I would not mix the two amps on the same pair of speakers for several reasons.
1) Each amplifier has a sonic signature. Chances are that if you mix these components, it may not sound proper.
2) Depending on the speaker, I have seen the grounds tied together on the crossover. This has the potential to create a ground loop in your system (making a higher noise floor).
3) If this is a HT setup, the 5th channel will be the same sonic signature you may use for the center. The Adcom could be used for the rears.

I would experiment and see what works for you...
Best of Luck!!

J_k advice is good. But since you already have the amps, why not just do a listening test yourself and determine what sounds best to you. Sometimes different amps can be a benefit. In my system I'm using Krell on the bottom and Levinson on the top. I don't much care for the Krell on the top, it's too harsh. I like it on the bottom end--it has more control in the lower octaves than my Levinson. Most poeple will agree that matched amps are the easiest and least problematic way to go for bi-amping. If you were planning a purchase I would give the same advice--but since you already own the amps--why not just test it yourself?

I am assuming that you are passively bi-amping (whereas I am actively bi-amping), so take my advice for what it is worth.
I partially agree with "abstract7", but I do use different Krells for both highs and lows.
I had first listened to which amp I thought gave the best overall sound, and then used that for the mid/highs and the other for the bass. The result is fantastic!
So, LISTEN, and make the decision based on that.
Richard