Pharmacologically, it's totally fine to mix them.
There is a certain cross tolerance developing between lsd and mushrooms, so if you take one first, the other substance may not hit as hard as it would normally. That being said, this is normally true when you wait till one of the substances has ended the effects to take the next one, but if you take them soon after each other like your plan, the cross tolerance shouldnt be an issue and you will have both compounds having significant effects.. Basically the trips will add up. Also, the cross tolerance is stronger if you take LSD first and mushrooms after.
Regarding the dose, it depends a lot on your previous experience. Have you taken 1.5g shroom before? Have you taken from this very batch before? How were the effects? What about the LSD, in that dosage, how does it hit you? What if you took more of each, how does it hit you if you take for example 2g shrooms or 200ug LSD, can you manage it ok?
For me 1.5g + 150ug would be nice, for others might be too much, overwhelming. Depends on your personal sensitivity. Since its your first time with this specific mixture, I'd definitely recommend you go to the lower end of your suggested dosage (0.5g + 75ug), better be safe than sorry
Will you be alone? Please be safe, dont take more than you are used to, and don't go near cliffs or dangerous places, etc.
Lastly, unrelated but.. You've been here in the forum for 4 years, but you are still under new member status. Why don't you write an introduction essay so we can get to know you a bit more and be able to promote you?