Check in / Boarding pass / Seats

Flying next weekend with a mate. We booked/paid for separately.

I am sure I have managed before, but I can't see a way we check-in together - so we get to sit with each other.

No way paying extra for seats together.

I know when I have done this before when sorting for me and the wife, but on those occasions I have booked the two of us - so details are under one account.

If we both check-in at the same time (different PC's) is there a chance of seat moves along to the next one along?

Thoughts/advice or is it a case of tough luck.

The only way would be paying seats. That way you can select a seat and then he has to select the seat next to you and rely no one took it before.
Without paying, I don't think that pressing the check-in at the same time changes anything.
You can always try though.
I fear there isn't any real way of doing that.
Better when it was free seating... Pros&Con's...

Enjoy your flight !