Setting up 2 power amplifiers to setup at once help?

Might consider using the 2801 to power the highs for your speakers and the 'sub' out from the 2801 into the Yamaha to power the lows.

Imo, it's not a good idea, you don't want to use a active crossover in the receiver and the passive crossover in the speakers at the same time.
Use a pair of 'Y' rca cable connect the FRONT R/L pre-out to both power amp input.

https://ae01.alicdn.com/kf/HTB14A.3RpXXXXa6XVXXq6xXFXXXU/ATAUDIO-HIFI-Single-RCA-to-Dual-RCA-Subwoof...

"Imo, it's not a good idea, you don't want to use a active crossover in the receiver and the passive crossover in the speakers at the same time. "

I am curious why you think this. You are simply sending a signal that has passed a low-pass filter to another filter that should pass that signal (if set low enough). 

I can see a situation where the signal sent from the receiver is too 'low' for the difference between the frequency ranges of the low to mid speakers, which is why I advised to dial it up till it sounds full. seems like this method would allow you to better control the low end. 

On the other hand I see no issue with doing it like you suggested (as I have done both) except it is harder to control the 'amount' of low end. 

Looking at the manual for the AVR-2801, which can be found via a link near the bottom of this page, it appears that in stereo mode (i.e., when a surround sound LFE signal is not present) the bandwidth of the sub-out signal will extend up to a frequency that is fixed at 80 Hz. (See especially pages 16 and 17). While the crossover point between the low and mid/hi frequency sections of the speaker is at 125 Hz. So using the sub-out signal to drive the low frequency amplifier would cause frequencies between approximately 80 Hz and 125 Hz to be under-emphasized or even almost absent (depending on how steeply the filters roll off), although the relatively narrow difference between those frequencies would tend to mitigate the issue. Also, as the notes in the manual make clear, there are a number of menu settings that would be involved in doing that, creating opportunity for error.

So I would go with Imhififan’s suggestion of splitting the front right and left channel pre-outs to drive the two amplifiers. Although chances are the physical spacing of the two RCA plugs at the end of the particular cable he suggested is not enough to allow connection to two separate amplifiers.

So it can be done without and active cross over? And unidentical amps. Considering that @almarg has mentioned that the gains are almost identical ?

Yes, it can be done, and **if** the Denon amp (as well as the Yamaha amp) provides adjustable input levels you would have a lot of flexibility in balancing the volume of the deep bass with the volume of higher frequencies. Which might be helpful in dealing with room effects.

One additional possible complication that occurs to me, though, is the possibility that one of the two amps produces an output signal having the opposite polarity of its input signal, while the other amp doesn’t. That’s probably unlikely, but if that is the case the resulting sonics would likely be poor (although no damage would result). If that is the case, though, you could compensate by interchanging the + and - connections on **either** the low frequency or mid/hi frequency section of each speaker (but not on both sections).

In any event, just be absolutely sure that the jumpers are removed from the speakers if and when you biamp. If they are not removed you would be connecting the outputs of the two amps together via those jumpers, which could damage either or both of them, as has been indicated earlier.

Regards,
-- Al

not worth it.....even if done correctly

"So using the sub-out signal to drive the low frequency amplifier would cause frequencies between approximately 80 Hz and 125 Hz to be under-emphasized or even almost absent"

Good point, which is what I was trying to emphasize here-
"I can see a situation where the signal sent from the receiver is too 'low' for the difference between the frequency ranges of the low to mid speakers, which is why I advised to dial it up till it sounds full."

This is exactly why I didn't try it with my system since my stereo  crossovers are pre-selected at 60, 80 and 100 Hz and my speaker crossover is set to 125. I had thought an AVR would have a more flexible range. My mistake.