The best way out is to switch to IKEv2. You can implement this workaround to allow multiple L2TP/IPsec clients to connect from behind the same NAT, but it only makes sense if you really need the L2TP/IPsec for some reason. The reasons why IKEv2 is a better choice are mentioned in that topic too.
Regarding the reboot of the other client - you may need to give some time to the firewall and the IPsec stack at the Mikrotik side to “forget” the previous connection, which may take minutes. So maybe it would be sufficient to wait for 10 minutes after the first client disconnects before attempting to connect the second one.