is there a way to make it so that i can play as gdi and nod at the same time on solo play? the reason y is because i like both of their super weapons but i like nod's defenses better.

Hmmm, it looks like you want a bit of power play. Well, there are 2 ways:

MOD
Some mods can allow you to control or merge 2 factions into 1. I used one though I forgot the name, sorry. 😞

GAME WAY
In game, you can either use an engineer or saboteur to capture the enemy's buildings. Most notably their construction yard & production buildings. This also applies for Scrin (Aliens)

Regards,
TWL

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On a single player mission you can also place what ever buildings you wish the player to have assigned to the player.

Too bad you can't do it the "Yuri Prime way"
In the Ra2 expansion you could just use Yuri Prime to "borrow" you friends'/enemies' MCV.

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I haven't tried it but I suppose you could use the script to transfer the MCV to the first walk on army.