I really think it depends on the rpers. While a collab is the easiest option if you are with an rper you trust, there are times when assumed or permitted bunnying is fine. 

There are several times I have had this scenario in the past. Much of the time it is collab or two part posting series. (Where one person types up their entire post, then pm the other the post - they add in their characters actions and pm back; done)

Yet there are those select few on the forum I trust without worry to bunny my characters to make things run smoother. 

Usually a quick pm even if you aren't close to the Rper can work it out quickly.