Are you able to tweak pics and add two adults in one pic?
Yes. To tweak or add/remove things just describe in the instructions how to change it in specific terms. To add people, upload the reference photos of the additional people shown in the way you want them painted in.
Yes, things and subjects from multiple photos can be merged together into one painting if the orientations are conducive to doing so. To see examples of this being done you can check out our gallery at https://www.instapainting.com/gallery/merge-photos