Only time you can't run the wires in the same conduit is if they were supplied from two separately metered panels.
What are the loads you are wanting to supply? If they are a well pump, septic pump or other motor loads, you will have a high inrush current (up to about 6 times FLA) which can result in a very high voltage drop on startup and be hard on motors. If for motor loads, you could need to up the wire size.
The NEC code book has a table in it that covers conduit fill.
This table from the 2017 edition of the NEC shows that you can pull up to 37 #10 wires in a 1-1/2" sched. 40 PVC conduit. I'd say the existing conduit is plenty big enough!
Have you check to see if the conduit is clear to pull the new wires? If it's been in there for some time, there's always the possibility it could be damaged. Doesn't happen very often (if installed properly) but it can.
I don't agree with solid copper being hard to pull in conduit in other than straight lines. It's done all the time out there. For long underground runs with bends or with heavy gauge wire (like say 200 amps for ex.) you just need to use lots of lube and "simply" pull HARD. There is no code rule on max. length of conduit but can only be max 4 @ 90 bends before needing a pull/junction box. They make various electric winches for pulling heavy gauge cables. A come-along can be used but is slooow. There are specs for max pulling tension on wires & cables. On 2 houses I've built, I've pulled 200 amp wire into around 200' underground runs in 2" conduit with up to 270 degrees of bends - bends were large radius type.

