Honestly, it really depends on how true and flat your existing roof is, as long as their are no possibilities of decking issues that must be dealt with.
Also, if you live in a cold climate, there is no practical way to install the Grace Ice and Water Shiled that most codes now require, so you would be getting denied that additional protection.
Additionally, by doing a lay-over, you are creating "Heat-Sumps" trapped between the two layers, which, along with any trapped humidity or moisture, will decay the new roof shingles even more prematurely.
Finally, as a long term vision, if you intend on staying in the home for about 15 years at least, (because that is about the longest the new roof will last as a lay-over), or if you may be planning on selling in about 10 years, you will have the additional cost of paying for a 2 layer tear-off to be able to re-roof the home at that time.
I don't know your personal age or your geographic climate, but that could be a major consideration in the long run. If either of those time periods are later on in your maturing, near retirement years, then, the additional costs associated in todays valuation of dollars, would be even more impracticle than currently.
Many home inspectors use the roof age and wear and tear and amount of layers as a key and critical factor in assisting their prospective buyer in negotiating a much better deal at the sale time of price negotiation.