Normally you're not allowed to mix service wiring and branch circuit wiring, but solar is special.
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.
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.