I have a kids seat on the back of my bike and pull a trailer. My kids are 5 & 2. If they both don't want to get in the trailer, I put my 2 year old on the bike seat & just ride with my 5 year old in the trailer.
My trailer is fairly roomy though and can still fit my 5 & 2 year old together.
I don't think that I would invest in an expensive solution that you will only use ~ 1 year.