Theoretically yes, if the train is not already booked out. Even if seats are available, leaving your reservation to the last minute could mean that all the window seats are already booked, and you might not be able to get 2 seats together.
I have just read your other posts, and it seems that you are travelling with your wife from Zermatt to Interlaken on 10/6 or 11/6. There are still seats available on most Glacier Express services on those dates, except for the 7:52 and 9:52 on 11/6, which are sold out. On the 8:52 on 10/6, there is currently only one pair of window seats left. The other services currently have plenty of seats, including window seats for 2 people sitting together. That could change over the coming days.
However, it depends on your route to Interlaken. The standard (fastest) route that the SBB timetable will bring up for the trip from Zermatt to Interlaken Ost involves changing trains at Visp. This trip is on regional trains, not the official Glacier Express, because the Glacier Express does not stop at Visp. You can't get off the Glacier Express before Brig.
So you have 3 choices for getting to Interlaken from Zermatt:
1. Take the official Glacier Express from Zermatt to Brig, and go to Interlaken from Brig. As you are aware, this will involve a compulsory reservation on the Glacier Express. From Brig to Interlaken, you could incorporate the scenic Lötschberger route from Brig to Spiez via Kandersteg, and change trains for Interlaken at Spiez.
2. Take the above route using only regional trains - no reservations required.
3. Take regional trains from Zermatt to Interlaken via the fastest route, changing trains at Visp and Spiez. This route goes through the Lötschberg-Basistunnel, missing the scenery around Kandersteg.