Difficult one, the actual AHT achieved is affected by a number of factors making an accurate glide path difficult so you have to work within a range;
1. Time of day can affect call length
2. Mix of calls,you state there are two skills, each may have a completely different call duration and if you only look at an overall AHT it is going to vary by the mix of call types potentially quite drastically.
3. Quality - some agents may quickly exceed the glide path target but at the expense of quality, others may fall short of the glide path but then have a greater focus on the quality element with the speed picking up later when they get more confident.
To set up a glide path you are going to need historical data to guide you or gut feel based on the complexity of the training required and how the agents behave in the training environment especially when they start to take live calls near the end of training. Also dont pick the high flyers because they provide a nice curve that meets the financial perspective, these are not the norm, you will need a mix to get the average curve.