@Riking is correct, with the Discourse Patreon plugin it is not possible to connect tiers from separate Patreon accounts to a single Discourse site. The way the plugin works is that it pulls all the tiers from a Patreon account to a Discourse site and allows each tier to be associated with a Discourse group. All members of a Patreon tier who have accounts on the Discourse site will be automatically added to its related Discourse group.
The plugin also provides the ability to login to Discourse via Patreon. Logging into Discourse via Patreon does not require that the user is a supporter of the Patreon account that is associated with the Discourse site. All that is required to login via Patreon is a Patreon account. This means that you could enable the
patreon login enabled Discourse site setting if you haven’t already done so. Your girlfriend’s patrons could then get access to your site by logging in via Patreon. With logging in via Patreon enabled, you could both provide links to the two Discourse sites on each other’s Patreon pages and promote each other in this way.