Dual X carriage printer - cooling fan

Hi,

I am working on my construction of a dual X carriage 3D printer. I have some thoughts about cooling fan behavior.

I have configured the Repetier firmware to use two print cooling fans - "fan" (i will call it fan 1) and "fan 2". I want to have fan 1 on 1st printhead and fan 2 on the second one. Unfortunately, when i switch printhead with "T" command fan control always goes to fan 1. So when the 2nd printhead is working its print cooling fan is off and all commands goes to the 1st one which makes no practical sense.

My idea is to change behavior of "M106" and "M107" commands to the following:
- When 1st carriage is active and no "P" is specified, the command affects fan 1
- When 2nd carriage is active and no "P" is specified, the command affects fan 2
- When "P" is specified, then the command affects the specified fan

I also propose the tool change "T" command behavior to do the following:
- Set fan speed on the next printhead to the speed of the current active fan
- Turn off fan on the current printhead
- Set active fan on the next printhead as active

I don't know how other slicing software deals with print cooling fans but Slic3r cannot control two fans. It just generates "M106" and "M107" without the "P" parameter.

I have also noticed one more thing when looking through the firmware code: Fan control commands for fan 1 are queued along with moves but commands for fan 2 take effect immediately. Maybe commands for both fans should be queued ? This would make more sense...

I am going to modify the firmware and change fan control behavior anyway, but first i want to post these ideas here. If they get accepted by the community i can make a pull request later.