Two cities one day

The answer to your question will be somewhat dependent on 1) what you want to see/do at each and 2) how you are traveling

We would most likely be traveling by train and would of course want to see as much as possible. I am thinking that Sintra would really fill an entire day. Especially in early July when it will be crowded. Right?

Yes, you're right. Sintra will probably fill up at least a day. Most likely you will not be able to see everything you want to see in a day. It's just time-consuming to get from place to place. (Hint: buy tickets for Pena Palace online ahead of time! Otherwise, be prepared to wait in huge lines at the entrance. Another option: visit the Moorish castle ruins first, and buy your Pena tickets there, where there shouldn't be a line.)

Plus, Belem and Sintra are not on the same train line.

However, Belem and Cascais ARE on the same train line, if you want to try to combine those into a single day (I did). Then again, I didn't see most of the stuff people tend to see in Belem, so I didn't spent a lot of time there.

Thanks Andrew...appreciate the comments.

I will echo the other posters that Sintra and Belem in one day isn’t doable. I will also add - taking a small group tour to Sintra is a smart way to go. The crowds are staggering in July. Most of us who live in the area don’t go there in the summer. But, having someone who can drive you straight to the palaces helps a lot. Yes, there is public transport, but it’s so overcrowded, it’s a pretty miserable option.

Thanks again for the hints.....appreciated.