The main consideration at this point is that you be with your children every moment they are in the tub. I would put a good full sized rubber mat in the bottom of the tub to help prevent slipping and sliding (which could cause the child to hit the hard side or bottom of the tub). There are also soft rubber protectors that can be put over the faucet to prevent the child from falling and hurting themselves that way. I have a duck that fits over our faucet, and the water flows under the duck's bill. The children love it.
But, after you've done all the protection you can of the tub, remember you are the #1 protection for your children. This can't be emphasized too much. It takes just a second for a child to get their face in the water and be confused enough to not be able to get up. And many children drown in an inch of water. So having said all of that, do be sure that before you start bathing your children you have all the necessary items near the tub, so you don't have to leave them to go get anything. This will include the soap, shampoo, washcloths, towels, their clean clothing... and any other items you deem necessary... as well as you either need to have a portable phone or your cell phone in the bathroom (or simply make up your mind that no phone call is important enough to require answering during bathtime). And if the doorbell rings, whoever is outside can wait or go away. If you find you MUST attend to anything else during the children's bathtime. wrap them in towels and take them with you.
Okay... now that I've said all those things about safety ... let me go on and say enjoy the bathtime with your kids. This can be one of the most rewarding times you can have with them. They learn a lot while playing in the water. Oh! And don't forget to put in some good toys.. boats, some cups for them to fill and pour, some 'floaty' toys, etc. Don't overwhelm them with too many toys at once. Two or three items for each child should be good with two children in the tub. Do expect to have to do a major wipe up of the bathroom floor after bathtime too. While you'll want to encourage the children to keep the water in the tub, you are going to have water on the floor and you may as well expect it.