Twisted pair does a excellent job isolating signals between the various pairs. You are not going to notice a difference between sharing pairs in the cable and multiple cables.
I prefer using an external POTS/DSL splitter. The phone company owns the NID and all the devices in it. An external Splitter typically has more filter stages so it delivers a higher level of isolation between signals. Having said that except in very special circumstances you will be unlikely to notice a difference.
Here is the data sheet for the one I'm using. It used to be made by Corning/Siecor but it looks like the design was sold to TII. The other advantage of an external Splitter is it includes the half-ringer test circuit. This allows the one in the NID to be removed. ADSL was designed to coexist with test circuit but if your signal is marginal every little bit helps.
Tom... thanks again for the good info, I was hoping I could use the same CAT5 line to run Phone/DSL, will save me from pulling more line. Will be pulling more CAT5, and updating the older phone lines in my house since I have notice some static on the line at times.