Site running on multiple versions

Hello!

We have a Drupal 6 site, and we have been debating upgrading it to Drupal 7. We had a consultant look over our site and give us an estimate regarding how long it would take to upgrade our site. They estimated that 27 weeks would be required to upgrade our site from 6 to 7. Because this process has been estimated to take half of a year to complete, people that are not me have decided not to go through with the upgrade right now.

I do not think this is good because when 8.x is released then 6.x may stop being supported. I also wrote a module that I would like to contribute, but I think I need to release it for 7.x and eventually 8.x. If our site continues on 6.x, then this is not likely to happen. We also will be adding more custom modules which we may or may not contribute, and I am concerned that the modules will not be of much use to newer sites running on 7.x or 8.x.

My workaround for this is to keep our current site as is, but to do our new development on a 7.x site. The user would access the 7.x site through the 6.x site and make use of the 6.x's site's users, roles, groups, etc. In theory, this would allow us to develop our new modules on 7.x and contribute them to Drupal.org for others to use, but also it would eliminate our need to upgrade our current 6.x site.

Is this a common practice? Has anyone ever tried to do anything like this? Is this a path worth pursuing?

I would love to hear your comments!

Thanks!