First, it depends on which solar panel you are trying to connect. It would obviously not be possible with the 40W panel since even at 100% utilization the max you will receive is 80W (in real life conditions it would be closer to 65W to 70W). When joining two 60W panels with the y-branch splitter, you should be able to get around 90W, potentially as high as 100W of output (again, depending on conditions).
Using the y-branch splitter cable, will I be able to harness approximately 100w, or is this combination not compatible?
First, it depends on which solar panel you are trying to connect. It would obviously not be possible with the 40W panel since even at 100% utilization the max you will receive is 80W (in real life conditions it would be closer to 65W to 70W). When joining two 60W panels with the y-branch splitter, you should be able to get around 90W, potentially as high as 100W of output (again, depending on conditions).