I personally have found that it has been a good idea to have 2 of the same printers in our business. Both printers (Mutoh 1624s) run the same ink, use the same parts and print the same. So, if 1 printer goes down, the other can pick up where it left off and colors and resolution looks the same - or when doing a large job the job can be split between both printers and achieve the same results with greater speed. As far as parts and ink goes - it really can't be more efficient, same dampers, same wipers, same ink, same heads, etc. We stock 1 of everything, and no matter which printer would need it its here and then we order a replacement as soon as that part is used up.
The only reason it would make sense in our business would be if 1 printer could produce something that the other couldn't - like maybe having a flatbed or UV printer, etc.