I'm booking a train ticket online from zurich to venice. for some reason when I try the "open" seating configuration and then click on window and aisle for two passengers, it changes it to window and window for both passengers. Why is this, and is it possible to change it?

You really need to give the name of the site that you're using for booking. Otherwise it's difficult for anyone to answer your question.

It is possible that the seat configuration is two seats facing two seats with a table in the middle or two seats facing each other with a table between them. If this is the configuration it is possible that you will each have a window seat. These are the seats we were given several times when I made reservations.

I used the sbb.ch website that someone recommended.
yes it looks like 2 seats facing 2 seats, but they dont let me reserve the aisle and window next to each other

Much nicer to face each other across a table as you would in a restaurant, and both have a window for the scenery, surely? I hate aisle seats!

<Much nicer to face each other across a table as you would in a restaurant, and both have a window for the scenery, surely? I hate aisle seats!>

I agree 100%

>>>but they dont let me reserve the aisle and window next to each other<<<

Perhaps if you weren't attempting to book it all on one ticket, you would have better selection. The Milan/Venice leg booked on Trenitalia will show you a seat diagram similar to when you select seats on airplane websites. The system is defaulted to automatic seat selection so you have to check the box saying you want to select your seats to get the diagram.