Last night this issue came up, not in my game but in another. There were two combats going on, one between some Tyranid Warriors and another involving Termagants. The player who's turn it was decided to do the Tyranid Warriors combat first. He wiped them out. Next, when it came to the Termagant combat they lost, and the question was, did they count as fearless because they were within synapse at the start of the combat?
My argument was that since the combat rules say to complete each combat before starting the next, the warriors are dead and the Termagants do not benefit from synapse.
The other side compared it to the Blood Angels where they gain Furious Charge even if they assault out of its range.
The order in which you do things is very important in 40k. Here's a similar situation that justifies my position. Let's say it's your shooting phase and I have a creature projecting Shadow In the Warp on your psyker. You choose to use another unit to shoot that creature, killing it. Now you use your psychic powers. Obviously, you can cast the power unhindered. Order of actions is very important.
What do you guys think?