Replace new lines with spaces using awk

I have a text file that I generated of all files in a directory. I'd like to use this file as input into a script that I have, but I need the text file to be formatted in a particular way to be parsed correctly. Currently the text file, which is a list of file names, is formatted like so:

A1_R1.fastq.gz
A1_R2.fastq.gz
A2_R1.fastq.gz
A2_R2.fastq.gz
A3_R1.fastq.gz
A3_R2.fastq.gz

I need the paired reads (files with the same name, but different RN values) for each sample to be on the same line, separated by a tab:

A1_R1.fastq.gz A1_R2.fastq.gz
A2_R1.fastq.gz A2_R2.fastq.gz
A3_R1.fastq.gz A3_R2.fastq.gz

Since I have >1000 entries, I am hoping for a method using awk or something similar to modify the file, but I don't have much experience with awk.