It's a common courtesy that, in a text-chat context, ends up being less courteous than just asking. If you "ask to ask", you are wasting my time with an extra interruption that I don't need. Now I either need to sit there and stare at "readhn is typing..." until you've finished your actual question (wasting my time), or I need to go back to what I was doing, only to be interrupted again in a minute or two when you finish typing out your question.

I understand that asking to ask is a courteous thing to do in general and follows the general rules of social graces, but when you do that, you're actually wasting my time and are being less courteous.

If you really must do this, just put the ask-to-ask in the same (initial) message as the actual question. Given that the ask-to-ask query is functionally useless anyway (what asshole would say "no" to that?), you get to appear courteous while also not wasting your colleague's time.