First, you can indeed use L2TP/IPsec and IKEv2 on the same router, as L2TP/IPsec uses IKE (v1) and the contents of the initial IKE (v1) packet and of the initial IKEv2 packet are distinctive enough that the IPsec stack could sort them out properly.
Second, you man not need to change any client configuration - I assume the reason why some moderator has subscribed me to this topic is that I have implemented a solution that allows multiple L2TP/IPsec to connect from the same site: http://forum.mikrotik.com/t/multiple-road-warrior-l2tp-ipsec-clients-behind-nat-solved/118206/1 .