How exciting! I know adding Lane to our family was a slightly different situation since she was coming from a rescue and was also a puppymill survivor, which comes with its own set of issues. I thought Rory would be a little unhappy having to share all her attention with another pup, but she could not have been any sweeter. Rory was very well socialized and active (like Jackson), and she attended doggie daycare daily while we were at work during the day too.
I'll tell you I kept going back and forth on whether or not I should add another dog with my biggest concern being how it would affect Rory. The second I walked into the rescue and saw all the yorkies pulled from the mill, I knew we were going home with one. My heart definitely wins over my brain in situations like this, so leaving your wallet at home might not be a bad idea. Lane sticks to Rory like glue, but I know Rory is thrilled to have a playmate and someone to play keepaway with at all times.
I agree the ideal situation would be if you could have a foster to adopt situation, but from what I gather of Jackson's personality is that he will be happy to have another companion around.
Good luck! I know it's easier said than done, but if something just dosen't seem quite right, it's okay to walk away. I know you know all the right questions to ask. On the other hand, I think you'll know pretty fast if Jackson and Marco are destined to be brothers!