I guess the mechanism is the same one that ensures that on startup the IPSec SA negotiation starts after ISAKMP negotiation is over. In case a state comes where the two SA lifetimes expire simultaneously, the ISAKMP SA re-negotiation process should get over before IPSec SA negotiation starts. However, such a state will not be reached since a new SA is re-negotiated before the old one expires, thus preventing a period where traffic has to be dropped.