The question for us should be when do we stop getting another?
There was 6 months between Barney and Jasper joining our household (Barney first at 11 weeks old and Jasper at 8 weeks old). 6 months after that Buster joined us, he was 10 months old, 12 months after that, Skye joined us at 8 weeks old and last but by no means least, 3 years after that, Basil joined us, aged 7 years old.
Barney really thrived on the company of Jasper, was tolerant of Buster, had a mega sulk for 2 weeks when Skye joined us and just raised his eyebrows when he saw Basil walk in. The cat moved next door, he'd had enough
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They interact great, there is definately a pecking order (Skye is top dog!!) and they miss each other if one isn't around for one reason or another.
I personally would recommend 2 rather than 1, but as aprevious thread states, it is down to personal circumstances, understanding the needs of your existing dog and indeed the affect on the purse!!
Good luck.
Lisa