Could multiple PCIe devices in one IOMMU group be added to different KVM VM separately?

TL;DR: you shouldn't but can try to fake/force the system into it

Having the devices in the same group is a deficiency of the HW/FW that you use for the use-case that you want to achieve. Guests using the different cards can effectively read/write each others memory (more complex in reality but a nice simplification). That is very much not what you want from virtualization.

There are non upstream accepted patches around this which can help you to fake/force splitting some of these IOMMU groups. But really - please read what it does and understand the associated risks here by the maintainer.

If you really want you can consider compiling your own kernel with the patch and then use these boot parameters which can help then to (fake-)split the devices:

pcie_acs_override=downstream,multifunction

If you have multiple PCIe slots, moving around the cards is a much better (and safer) way to achieve the same.