Biamping with 5 channel home theater amp

Will not be a problem if you have two sets of outs on your pre.

Splitting the signal might degrade somewhat but no operational/safety concerns exist doing that.
As Eldarado said, 2 sets of outs on the pre IS better.

Many preamps with 2 sets of outputs are simply single outputs with an internal Y connector. In any case, it is just fine as long as the input impedance of the power amps is high enough.

Kal

If, by chance, you have a Sunfire Cinema Grand, it's very easy to do using just a single pre-amp output, as explained here...
http://www.vxm.com/21R.116.html

That's precisely what I did for driving ML Aerius i's... worked great (however, I've since moved up to ML Summits).