How to concatenate two fasta sequences by py
First of all, those are not fasta sequences. Fasta is a specific format, and requires a &gt; followed by a sequence identifier on one line and the one or more lines of sequence.
Assuming you really have two files in the format you show, you can do:
$ awk -F: '{ gsub(/ /,""); if(NR==FNR){bbone[$2]; next} for(b in bbone){print b$2}}' file1 file2 
GTCATGCTACGTGCTACGTGCTCGTCGCGCCGCTAGTGTAGCTGCTACTCCG
ACTGCGTATGTCAGCATATAGACCGCCTCGCTAGTGTAGCTGCTACTCCG
CGTAGCTAGCTAGCTCGTGTCGTGATGCTACTCCGCTAGTGTAGCTGCTACTCCG

The gsub(/ /,&quot;&quot;); just removes the extra space you have in file1 after each :. If you also want to have the sequence names as you show, use:
$ awk -F: '{ gsub(/ /,""); if(NR==FNR){bbone[$2]="Backbone"NR; next} for(b in bbone){print bbone[b]"+"$1":", b$2}}' file1 file2 
Backbone3+sequence_1: GTCATGCTACGTGCTACGTGCTCGTCGCGCCGCTAGTGTAGCTGCTACTCCG
Backbone1+sequence_1: ACTGCGTATGTCAGCATATAGACCGCCTCGCTAGTGTAGCTGCTACTCCG
Backbone2+sequence_1: CGTAGCTAGCTAGCTCGTGTCGTGATGCTACTCCGCTAGTGTAGCTGCTACTCCG

If you really need to do this in python for some reason, please edit your question and show us your python script so we have something to work on and expand. I really don't want to write a whole python program for something so trivial.