2 broken spokes same side, can I replace with one from the other side?
Can you? Sure!
Should you? Well...
You say it's a mountain bike - what kind of riding do you do? If you're mostly doing commuting on smooth roads, you should be able to get away with it through careful riding, avoiding curbs, pot-holes and other harsh knocks. If you're riding off-road on even moderately bumpy terrain, I'd say that's a prescription for a wheel failure which will end up in another accident, and this time it may not be just the wheel that ends up broken.
As far as spoke wrenches go, when I last really worked on a wheel, the top of the nipple had a small slot in it where you could use a slotted screwdriver. It appears that they still do:
Shamelessly stolen from Bike Parts Place.com
You can remove the tire, tube and rim tape then use your screwdriver in this slot. I'd suggest that when you order your spokes you would want to order either a multi-size spoke wrench or a set of single-size wrenches (I just picked up a Parke Tools multi-size one last night for $10, the set was about $15) as that will make quick tweaks to truing much easier in the future. I'd also suggest ordering a few extra spokes, just to have them on hand should this ever happen to you again.
Since it's your rear wheel and that takes the majority of your weight, I'd suggest that you'd want to temporarily replace both of them with spokes from the front. Where from the front you'd want to pull them, I'm not certain - I'd hope someone else will chime in on that. Pulling them from right next to each other increases the weakness at that spot, but makes it easier to maintain true, while pulling from 180° from each other spreads the weakness, but makes it more difficult to true up.
As a side note, I'm curious where you are that you've got a bike with 29&quot; wheels - I certainly don't know all there is to know about bikes, but that's a diameter I've not heard of before.