The problem would become visible if transactions in period 2 would be smaller than 1:2 versus period 1.
And in a scenario that the number of payments remains stable, each payment would be verified by 2 senders, right? Is that sufficient to stop hackers?
Some people suggest that at times when transactions are reduced there could be attacks. Look at this:
However the creators say in their paper that that would not happen. Who will be right? mmm... We need to investigate further.
This comparison between IOTA and Byteball may be helpful as well.