Merge several .ab1 sequence files into a single FASTA file?
In general you should use a base calling algorithm to generate the sequences from the chromatogram and not directly convert it to fasta (Courtesy: Sven [SEQanswers] ). As mentioned in the link, phred and TraceTuner are popular base calling software that can generate a fasta output.

The software mentioned by The Nightman can be used for converting .ab1 to fasta. You can also try this BioPython module called abifpy. You can easily read each .ab1 file, using a python script, and write the sequences in fasta format as a single file.

Combining multiple fasta is quite trivial. You can use cat as mentioned by others.