Solar 240 VAC source and load wires sharing a conduit
Normally you're not allowed to mix service wiring and branch circuit wiring, but solar is special.  

Derating

It's almost never a problem with 2-3 circuits, but when you put 2+ circuits in conduit, you must pause to do a "derate" calculation.  Other rules prevent you from using wires at their thermal maxima, but you derate off of that.  For THWN-2 wire, that is the 90C column of table 310.15(B)(16).  So for instance if you're running 8 AWG Cu wire, your 90C number is 55 amps.  

Your derate comes out of table 310.15(B)(3)(a).  In all split-phase 120/240V circuits, there are only 2 wires that count, so you are always 2 wires per circuit.  So 2 circuits is 4 wires, which calls for an 80% derate.  80% of 55 amps is 44 amps.  

You then round up to the next size of breaker.  They don't make 45A breakers, so you go to a 50A breaker.  Lucky you! 

This does not liberalize any other rules.  If this was #8 Cu NM cable, that's only allowed to run at 60C, so it is limited to 40A regardless.

Up to 4 circuits per conduit (70% derate) is fine for 15-30A circuits. However for 50A and above circuits, it starts to pinch. #8 THWN-2 can only carry 40A.  If you ever see installations with many conduits next to each other, that's why. They are keeping it to 3-4 circuits per conduit. 

Can you use the 90C column if your termination is only 75C? Depends if the termination is inside the conduit! (unlikely to be legal to splice in a conduit, but perhaps in a raceway, gutter etc.? Anyway if it was a 60C splice, yes, #8Cu would derate off 40A at that point.) If the splice is outside the conduit, then conduit/raceway derate rules are inapplicable and you apply whatever rules exist for the space the splice is actually in.  

Branching

Anywhere wires split, there must be an accessible point, such as a "T" conduit body or a junction box.  (Places where splices happen have much larger cubic-inch requirements; junction boxes are sized to allow splices, conduit bodies are not).  

Every accessible point most remain accessible in perpetuity. For instance if you get a mind to deck over your attic to use it for storage space, you better install a junction box so it'll be flush with the ultimate floor, not bury a conduit body under flooring. 

Also you can't have more than 360 degrees of cumulative bend between accessible points; for a novice DIY puller, even 180 is ambitious.  I prefer 90-135 at most.