Transparent forms

You need to first set the opacity to 100% and blending mode to normal.

Then group the bottom left and right circle. Set the opacity of the group to 50% or whatever

Then group the top left and right and set its opacity to 50% or whatever.

In the appearnce panel the "Group" needs to be targeted. Then set the opacity.

This will need some jump start into Illustrator. It's not really beginners' stuff. Please read:

How to adjust appearance attributes in Illustrator

I think this is what you are looking for:

All four circles have the same opacity (a randomly-selected 73%, if that matters). I grouped the two on the left, grouped the pair on the right, then grouped the two pairs. In the Appearance panel I clicked on Knockout Group, either once or twice, as needed to get the check or tick mark to appear.

Please let us know if this is not what you want.

Peter

well, when you overlap some semi-transparent objects, the total opacity increases even up to 100%.

What do you want to see? Should it be overlapped or become a united form with the same opacity through it?