Hi,
It is mandatory tyop have one end as static so that the remote end could initiate the ike negotiation to the static end.
Imagine the suituation when both the ends are dymamic and are not aware of the ip's , what destination ip should the device put in the ip header when traffic is initiated.
Moreover the suggestion that you have given to use the dynamic ip's may bring up the vpn for a while but will break as soon as the ip changes ( But make sure that ip doesn't chnage very frequently , before the ike negotiation completes and if ip chnage sthe negotiation will fail.)
Regards
Hemant