I currently have a -1.5 degree angleset cup installed on the bottom of my bikes headset and a 0 degree on the top. I would like the head tube angle a little slacker so am wondering if it is possible to use a -1.5 degree cup on the top as well which would give -3 degrees of HTA. Am. I dont think there would be an issue but wondering if im missing something.
You can 100% physically do it. Just make sure the angles of the bottom bearing and race will cope with the shift. ie that it works with the change, and is not designed to only work with that specific top one.
Now, you haven't mentioned the bike or the type of riding, or the reason for why you want it slacker.
As a general rule, if you make a bike slacker (from it's intended angle), it will make turning less snappy and stright downs more stable. But obvs whether this is a good thing, is subjective to the bike, the type of riding and the riders ability and preferance/style.
Worst come to worst, you don't like it and you can just switch the headset back. Just keep the original one just in case.
Other things to consider, is how the slacker angle will affect the rest of the bike, both in geometry and in physical forces. For example, if you slacken a bikes, which is not inteded to be, you increase the chance of the headtube splitting from the frame. You also decrease the uphill riding efficiency, lower the bottom bracket hight and the way the front fork travels might change too.