Dan Durston shared the following advice on Trek-Lite forum on how to pitch the X-mid if the space is tight.

There are several options here including ways to do it with the inner not exposed.

First, there is a 'false method' where you don't actually collapse the vestibules - you just roll back both sides of the door. The large side of the door is rolled back, while the small side of the door can be folded around the corner and tied/clipped to the loop on the bottom hem. This is a quick easy method that would be handy if the weather is nice but you have something blocking your site along that side (e.g. a boulder). With this method the inner is still protected from vertical rain, but you wouldn't want to do this in blustery conditions since the mesh sidewall is exposed. With this method the corner stake and full end wall are still in position, so it has the advantage that you don't need to use any guylines.

Now if you actually want to collapse the vestibule, here are the steps:

Step 1 - Deploy the Peak Guyline
The first step is to deploy the peak guyline. Normally the ridgeline tension is held by the corner stake at the vestibule, so if you want to unstake that corner and collapse that corner (or not stake it in the first place) then you must deploy the peak guyline on that side to hold the ridgeline instead. If the weather is mild, you can stake the peak guyline to the same position that the corner would have be staked at. This lets you clip the corner of the floor to this stake to hold it in position and you can easily restore the vestibule if you wish).

Step 2 - Anchor the Floor
Next, you need to anchor the corner of the floor somehow. Normally the floor is clipped to the fly corner that you just collapsed, so you must do something else to anchor the corner of the floor. Your options are (1) stake the corner of the floor, (2) clip the cord that normally holds the inner out to the peak guyline (if you staked it near where the corner normally is), or (3) connect the floor corner to the extra stake point along the hem near the floor corner, and stake that extra hem stake point.

Step 3 - Tidy up the Vestibule
Now you have a collapsed vestibule and a taut floor, but you want to do something about the loose fabric so the vestibule is not just hanging there. Your options are:

1) Reposition the corner stake by staking to the end wall stake point (that you might be using to anchor the corner of the floor). This is very similar to the Stratospire method where the inner is protected and it gathers up a lot of the loose fabric, but there is still some loose fabric.

2) Leave the corner stake point unstaked, but pull the door stake point outwards and stake that out. This is an interesting one. It changes the shape of the base of the tent from a rectangle to a hexagon (if you do both sides). From the arial view it looks like it doesn't help much because the vestibule is not collapsed, but it would help if the problem at the corner (e.g you are trying to fit into a round site) and secondly, when you do this the door stake is going to float quite high off the ground (by about 30 cm). So it lifts the bottom edge of the vestibule way off the ground such that it can float over top of small bushes much better. There is still some loose fabric but it is far away from the inner. So this method can solve the problem while keeping the inner protected and minimizes issues with loose fabric.

3) Roll up the corner of the tent. You can roll up the side wall and end wall, and then tie that roll securely using the tail end of the peak guyline. This avoids loose fabric, but does leave the inner exposed. You could easily unroll it later and stake it to switch to method 1 or 2.

There’s also a video of the 'skinny pitch’ with X-mid 2p Pro