Totally possible to use both cards
You can connect both directly to your motherboard, or even better, you can buy a riser cable
On my personal desktop I have a 5700xt directly connected to the motherboard and a GTX 1660 connected with riser and I put on the side of my pc
I use both to mine and if I need to use some program that demands more power, I just close the miner of 5700xt and let 1660 mining alone
I suggest you buy another motherboard for that super 2070 GPU, if you try using the card and mining with the 1070ti on a single motherboard it will affect the 1070ti you are using to mine by marginally, can cut off hashrate by 50% or even make your PC shut down
There's no need to buy a new motherboard (with a new one you'll need a new CPU, ram, ssd, PSU etc), it's a waste of money and electricity
You can run 2 miners, one for each card and won't affect any other card for mining
If you are mining and your PC is lagging, please post here all info and we can help you
I use here: Ryzen 1700 - Mining XMR
5700xt - mining ETH
1660 - Mining RVN
HDD 1 TB - Mining MASS
I can use the PC all day without any error, I just decrease the intensity of CPU miner when using