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I found the same problem. Here's what I do. On one side, I place the rivet in the hole, and then use the hand riveter to lightly set it with one pull. The rivet is lightly set, but the head isn't expanded yet. Then I put the rivet in on the other side, and set it fully. Now both rivets are in their holes. Then I set the second rivet fully.
I had to drill out a few rivets on my first flap before I came up with this method. I did this for the ailerons, too.
I had same issue with my 650. Learned from other builders to drive mandrel out of rivet, cut body of rivet to a shorter length that will insert in rivet hole and clear opposing obstruction and reinsert mandrel in rivet. You may then place rivet in hole and pull with head fully seated. Do not know if this meets specs, but is how many have met challenge of opposing rivets. Jerry