I made out a road trip this June. it's a awesome place there r several places to visit in meghalaya itself. Better u land n hire a car/SUV to drive around. Shillong, Cheerapunji, mawlynnong, Dawki r few places to go around in Meghalaya
Make out on state at one time. otherwise it would be hectic for u to travel around sikkim n meghalaya. Both r part away. if u r going for vacation thn visit sikkim or meghalaya.
U should either visit meghalaya or sikkim. only one place.In meghalaya u can go to cherrapunji, dawki, laitlum, mawlynong. It would take around 5 days.
Both are different and distant places. suggestion would be to spend atleast 7 days in Sikkim alone. Places not to miss in Sikkim :
Gangtok -City sightseeings
Himalayan Zoo to see Red Pandas
Lachen - Indo Tibet Border
Lachung - Yumthang Valley