can you put two usa maps onto one sd card
Is this a query about adding Garmin suitable Openstreetmaps to an sd card?
If so, I expect you can if the .img maps fit and the names are different.
Be sure to have only one map enabled or you will get unexpected results unless one is just a transparent contour map to overlay another map.
You can use http://www.javawa.nl/jdm_en.html to edit the map names.
Card size is model dependent
For example: https://support.garmin.com/faqSearch/en-US/faq/content/UsL6MuMMCM8PpgAhUUCqp7
And for outdoor models: https://support.garmin.com/faqSearch/en-US/faq/content/U8uZN9B2OM0AImFS8HFIxA