Hi my name is ***** ***** I will assist you today.
Firstly you need to remove the window pane, ensure you have eye shields and wear suitable gloves.
I would start by scoring around where the wood touches the glass to remove any silicon or paint which may bond the wood to the glass, then remove the the wood trim either with a wood chisel or claw hammer depending on the amount of wood rot you encounter, Once the surround has been removed the glass should be able to come out with minimal force, it often helps having one person pushing while the other person guides the pane out.
Next you need to remove the wooden frame, the best way to do this is lift out the interior window sill which should expose the brackets and fittings, you may also need to remove part of the interior wall cheek, to remove any brackets holding the frame in place.
Once the wood frame is removed its then a case of taking the uPVC frame and offering it up to the gap, make sure its square, and then securing the brackets into the walls.
If you are considering bonding two uPVC units together, the best thing to do is use a combination of silicon and screws, to ensure it stays strong and water tight, you wont see the screws as they will be by the windowpane. You may also want to use an extra bracket top and bottom to add extra security. Once the uPVC frames go in, its a case of using exterior silicon to fill any gaps, packing the interior frame and then installing the window units into the frame, squaring those off and fixing the trim to hold the pane in place.
If you need anything clarified please let me know.