The simplest way to implement this would be to use a SNAT address from the IPSec tunnel local phase2 selectors, as this already works for you. If you would like a different solution, let me know so we can discuss it.
Without SNAT, you need to make sure that 2 SSLVPNs are using different subnet for clients. Then you can add the SSLVPN subnet into ipsec as a source selector with correct destination selectors. If you are using wildcard selectors, you just need to make sure that on FortiGate B you have correct route for SSLVPN subnet from FortiGate A via tunnel (to avoid RPF). And then firewall policy on FortiGate A from ssl.root towards ipsec tunnel is needed. If you did this (or with SNAT) and it is not working, run debug flows on both devices and check where the traffic stops.
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