Choose the subpanels for the one parameter that matters: PLENTY of breaker spaces. The amp rating of the subpanel does not matter. Really. Spaces are cheap, running out of spaces is expensive.
Well, OK, the amp rating of the panel is a redline which you must not exceed. If running it at redline is your style, sure.
You are being unclear on whether the shop/guesthouse is an attached building (connected by a hall or breezeway) or a detached building. If it's a detached building you are not allowed to run two separate power feeders to it. You must consolidate that into a single feeder. If have thru lugs or can get subfeed lug kits for 200A, you might as well run 200A to one subpanel, then 100/125A to the other subpanel.
Can I use 2ga wire for a 90' run fro main to sub?
For 240V voltage drop is not a consideration at that distance.
Aluminum is perfectly safe at these large sizes, so there's no earthly reason to pay 3x as much for copper. As such we tend to assume we're talking about aluminum. Ampacity of wire comes from the 75C columns in Table 310.15(B)(16). Unfortunately many people read the wrong table.
2 AWG aluminum is good for 90A.
1 AWG aluminum is good for 100A.
1/0 AWG aluminum is good for 120A (no 120A breakers exist; use 125A).
4/0 AWG aluminum is good for 180A. However if your entire electric service from the utility is 200A and it serves a dwelling, you are allowed to use 4/0 for any service wire or feeder.
250 kcmil aluminum is good for 205A.
Can I use one 2" sch 40 pvc for all wires and then split to each sub panel?
If it's an outbuilding, no. You must have ONE feeder (4 wires) feeding both panels. Make one subpanel 200A and feed it with the largest cable you can get off the largest breaker or subfeed lugs you can get. Feed the other subpanel from that subpanel.
If it's an attached building, then you can run both feeders together BUT... if you do, the wires' capacity are reduced for thermal reasons.
2 AWG aluminum: now 80A
1 AWG aluminum: now 92A
1/0 AWG aluminum: now 108A
2/0 AWG aluminum: now 120A
Can I run one ground and one neutral that would be shared by both sub panels or do I need to run two neutrals and two grounds?
Ground can be shared. It only carries current during momentary fault conditions. Both sides are unlikely to have a huge fault at the same time, are they?
Neutral: No, no, no! The neutral is the normal current return so it needs to be thermally rated for the job.
You know, people see neutral and ground connected at the main panel and think "they must be the same thing". No, they're not. Totally different jobs. Neutrals go to neutral. Grounds go to ground. That connecty thing is a "Neutral-Ground Equipotential Bond" and if I were king, it would be a special pink wire, so people understood it was special. As things are, they let you use "the netural bar" as the N-G bond.
Is it correct that I only need to drive two ground rods 8' deep, 6' apart for both sub panels? Thanks for the help!!
You don't need any ground rods at all if it's an attached building.
If detached then what you say is fine but the farther apart you put the ground rods, the better they work. If you care about them working. But if detached you need one large feeder.