That's going to depend a lot on how much traffic and how resource intensive those sites are. However, you could probably host them both on the same server. I'm not sure exactly how much a small droplet can take, if anyone has that information I would sure like to know as well.

Not really an answer, and I'm still very very new at this, but the speed of the $5,- droplet/server just feels good :)

Even if a 1gb (10$) is recommended for wordpress, and the themes I installed and stuff, I really haven't noticed any problems.

Over all, it's the traffic that counts. How many people in total are using your websites at any given time?
It would be easy to manage even 10 websites if your traffic is low (like for small local businesses such as my hostel). Just keep your eyes on the memory usage and if that stays low enough at your peak-hours/days, you should be fine to add another website, and another, and host some images for your family and friends in high-res instead of facebook-cropping, etc, etc.

Simple answer: Yes you can.

Try to use a service like Cloudflare to low the load on your server and sites.