Mike K5ESS referred to posts by Raj VU2ZAP for an online generator that can provide two tones for testing your SSB output:
Another alternative is to download the application f-Generator for loading on to your cellphone. The free version will give you a single tone, but you will need to pay a dollar or two in app to get two tones.
Mike also alerted us to an article in ARRL’s March/April QEX by Phil Salas AD5X describing a two tone generator fabricated from two Wien bridge oscillators that are available from Ebay. I’ve put together a PDF that describes the generator for those that might want to build one but don’t have access to QEX.