Discount Rate on Synergies?

Thanks in advance, WSO hive-mind. Two questions that I'd welcome some input on:

1. When calculating the PV of synergies, how do you arrive at the target and acquiror's blended cost of capital? Weighted by revenue, or weighted by FCF contribution?

2. If a giant company is buying a tiny one, the discount rate on the synergies is going to be only slightly lower than the acquiror's standalone WACC (due to the relatively tiny contribution of rev/FCF from the target). But doesn't that discount rate understate the risk / probability of achieving the synergies?