If you keep that feed system you have to go all the way to 3/4 wavelength per side for the impedance to drop enough to again work effectively with coaxial cable. That is a length three times it was compared to the original half wavelength dipole. That can work, as people who use 7 MHz dipoles on 21 MHz can attest.
* Some QRP email list discussion on extended double zepps
* W5DXP on the half extended double zepp