Yes, I did find one as you described, the RF-390 30 - 512 MHz. It has two connectors at the bottom for the low and higher frequencies. I have not installed it yet, so not sure on performance.
Here is the spec sheet, so if the frequency you are operating on falls in the -6db of gain, which is likely as the higher frequencies usually have the best gain in these types of antennas, if your transmitter has 50W of output you will have the effective radiated power (ERP) of 12.5W and that is referenced to a 1/4 wave ground plane which is considered "unity" gain.
On the other hand if you had a +6db gain antenna which is common for amateur/commercial antennas then you would have an ERP of 200W
Also you will notice the SWRs are 2.5:1 @ 90-512MHz this will cause most radios to reduce output power significantly so that 50W transmitter may only output 20W, (this would give you an ERP of 5W) even worse at 30-90MHz