"to awasete" not "o awasete"

I'm curious about "to awasete"

と併せて

It seems to mean something like "both" or "all of".

I have an example but I can't post it.

The first example I could find from Google was

国民やアジアの人々の苦しみと併せて調べ、日本の侵略戦争の実態をとらえる

I just wonder why there is a "to" rather than an "o" there though.

.