Brynjolf Irontooth said:
Tim has it right that you draw from the blip pile, but I'd add one thing. Page 14 uses exactly this event card as an example. In addition to what Tim said they specifically state that a player must choose a valid target for event card abilities if possible. Using your card as an example they state that a player must choose a marine that would spawn the most genestealers possible (two in this case). So you can't choose a marine that would result in spawning only one genestealer when the you have another eligible marine that would spawn two.
If you have a left blip pile of 1 and a right blip pile of 4, and you have to spawn 2 GS, then you have to spawn on a SM's right as choosing the left pile would only end up in spawning 1 GS. If both piles would be 1 thick, you could choose both, only spawning 1 (sadly enough ) and then offcourse travel at the end of the event phase (as one pile is now 0 thick).
"Whenever a Genestealer is spawned, the current player takes the top card of a blip pile and places it face up in the appropriate position and side of the formation." I found it. But I don't think that it means you would have to place them absolutely on a SM's right.