So you could get another SM58 or even an SM57 (which Christian rates highly for general use) and use those as a pair.
Most the time you will find however that people tend to use condenser microphones for piano recording. An obvious consideration there is to make sure your interface has phantom power as most condensers need this.
For Family Piano I used a pair of Sontronics STC-1 which I borrowed from a friend. I think the results came out very nicely (especially after noise reduction - using RX7). They would be in budget and if you don’t mind a slight scratch or imperfect printing then you might be able to pick up a pair for about £200 from their b-stock.
Another option might be a pair of Aston Origins, which would come in about £400. I’ve heard very go things about Aston Mics. Personally I’m waiting until I have budget to get the Aston Spirits (about £300 each), which are multi-pattern, but the cardiod pattern of the Origins would be pretty good for a piano.
I’d love to hear what mics other people used when recording their pianos.