Winter bike

You don't actually say how the Decathlon is too big, f you can easily swing your leg over the top tube a bike is rarely so.

Too many people giving poor advice and selling bikes that are too small and then end up having to flip the stem and/or buy a much longer stem and pushing the seat all the way back and half a mile of seatpost sticking out. Fine on an out and out race machine if that's your thinking but for ordinary folk or a non race machine it's dumb when the next size up would actually suit better and take fewer adjustments.

As others have said, mudguards first and foremost and simply get the position sorted so you don't feel it's too big, just set up like your new bike with maybe a bit more height at the front and a cm or so shorter nose of saddle to handlebar length to give you a more relaxed winter position when you're not wanting to be on the nose. Maybe the bigger bike actually gives you this already? 

Winterise the BTwin with mudguards, reflective tape, suitable tyres and lights. If you're not rigourous about cleaning and lubing, winter commuting can eat drivetrain components, and I assume the groupset on the BTwin is cheaper than the one on your Canyon.  I have a winter bike with 10-speed Veloce on it for this very reason.

Re sizing, how "too big" is the BTwin; would a shorter stem make the fit closer to your Canyon?

And don't forget - mudguards are cool.

If you can, put full length mudguards on (not the quick on/off variety); it helps keep the winter cr#p off you and the bike. It may be a bit 'non-roadie' but I reckon the canny cyclist knows that it'll pay dividends by reducing wear on components.

Agree with Shay but there's 2 approaches:

Set it up exactly the same - some simple pedal to seat measurements and seat nose to bars will help.

Set it up more comfortable - I'd still look to start similar but then adjust as you see fit, maybe less aggressive front end as you're less likely to be getting on the drops and minimising drag for a sprint!

As long as the distance from the pedal to the seat is the same there really is no detriment in changing from one bike to another.

A winter bike is often used for steadier riding and so having the bars a touch higher and/or the reach a little shorter is quite common too.

On a winter bike you might have the saddle a little further back from the bottom bracket more like a touring bike would but it isn't important.

A bike fit can get you into the most efficient position but that is not always the most comfortable and on a winter bike you are not trying to break any records so it really isn't necessary.

Don't know about the fit - I switch between very different bikes weekly and get away with it - but I'd watch your chainset.  I rode through winter a couple of years ago on a Dura Ace groupset / Chris King hubs (on a Litespeed): the Litespeed survived but the chainset was destroyed - chainrings, cassette, and chain all needed replacing - and the rear hub cost GBP 270 to rebuild.  It's the wet, sandy/gritty, grey paste that grinds everything down.   Save the Canyon for dry days  

I have a Canyon Endurace and it went from one winter fine. Get some raceblade mudguards and you are sorted. If you have nice wheels then just use your other bikes wheels for winter. I would also put a cheap chain on in maybe

As long as you don't switch between bikes during the winter I think you should just set up BTwin for winter. It might take a couple of rides to adjust to the different fit and the same again when you switch back to the Canyon in the Spring. That's me being frugal, especially as you say have only recently bought the Canyon. I'm sure someone with more knowledge will tell you that you will destroy your back riding an ill fitting bike in the cold.