... I would like to build a more efficient antenna for 40 and 20. ... Jerry
One of the keys to effective and successful vertical antenna systems which is spelled out in the ARRL Antenna Handbook and ON4UN's text is two fold: 1) full 1/4 wavelength radiators, and 2) adequate and properly installed radial fields. (a third is having your antenna system away from nearby objects that may substantially interact with it such as close by metal structures.) A quarter-wave radiator with 'good' radial field is hard to beat and has a number of positive attributes including manageable height on 40M (33') and a feedpoint impedance that may match your 50 ohm coax without matching transformers installed at the feedpoint (a RF Choke is recommended).
IMHO, if you have room use a quarter wave vertical 40M and install an Unadilla 20M trap in it. The antenna height will be approx 30'. Try and lay down at least two dozen radials, around 30' each is adequate (more gives minor improvement). I've used this setup successfully. This setup is less problematic than your proposal of using 5/8 wave on 20M, and 3/8 wave on 40M, both mechanically and electrically. In the vast majority of settings, there is no clear-cut superior
advantage of the 5/8 wave over the 1/4 wave on the HF bands. Perhaps someone can do a quick model for you and compare the two designs.
GL, 73, Rich, K3VAT