To double cross does not mean to cross yourself twice.

" double cross（出卖）" 不是交叉手指两次的意思。

They become cross with each other -- and with you! They both think you tried to double cross them!

他们彼此争论（cross with each other）了起来——也和你争论。他们俩都觉得你出卖了他们！

I can't believe you would double cross me, just as I was about to double cross you.

我不敢相信你会出卖我，就像我正要出卖你一样。