Currently using a 4x8 tent in a garage that is dusty. would getting a extra intake be better for my plants?

it would allow you to balance out the power among your fans, but you'd want to hit the same exchange rate.. Once per 2-3 minutes? is that what 'they' say? Take volume of tent and divide by 2 or 3 (from above). then account for any loss due to ducting, bends, filters etc... always buy larger than you need and try to shoot for 50% power. It'll be way quieter and fan lasts longer. If you have filters, you'll want a slight negative pressure to keep the stank in. Using 2 intakes would help the exhaust push more air at same settings as before, too. might be able to tweak it down a little after adding a 2nd intake, if you do.