For your easy runs you are better off running 1 continuous run per day rather than breaking it up into 2 runs until you have reached ~ 60 minutes in that one run. Once you have reached 60 minutes then often you would benefit more from adding a double rather than extending the easy run.
A few tips:
- make sure any run you do is 20 minute or more
- once you have hit 60 minutes in an easy run add on by adding a secondary run, starting those at 20 minutes. Gradually add then number of days per week you do a second run and then work to increase the secondary runs.
My advice for you would be to continue to slowly build onto the 3-4 miles you currently do until you hit that 60 minute level before exploring of you would like to consider doubles.
Note: these tips/guides are for easy runs and not stress workouts. Stress workouts such as long runs, tempo runs or intervals have their own rules and guides based on what system is being targeted to be stressed that day.