Can we speed double on RB75GR3 with the same ISP (200 mbps + 200 MBPS) = 400 mbps?

Some applications may be able to take advantage of such an arrangement, but in general no.
The advantage of two ISP connections, is added throughput available to all. So, for example 50 users would be sharing 400Mbps vice 200Mbps.
If the ISPs are different (not same company or source), then you have a backup or failover capability as well.