Short answer: my advice would be to buy a new antenna - they are not super expensive, and certainly cheaper than replacing both the antenna and the radio.
Antennas need to be resonant at the transmitted frequency in order to reduce the SWR and maximize the field strength. They achieve resonance by being a certain length, or by using a loading coil with a shorter antenna. Loading coils are commonly at the bottom of the antenna and are not needed with a 6ft VHF antenna, so by losing the top 2 ft, you have just shortened the antenna, making it resonant at a higher frequency. In round numbers, if it was resonant at 160 MHz previously, it could now be resonant at about 200 MHz.
You may have noticed that the radio still receives ok. Resonance is much more important for transmitting.
A simple measure of the effectiveness of an antenna is an SWR meter. SWR readings are given as 1:xx, where the important part is the xx.
An antenna of any length will have a frequency where it is resonant, and the SWR at that frequency should be close to 1:1. At higher or lower frequencies, the SWR increases. A good marine VHF antenna will give an SWR of less than 1.5 on all channels. Most transmitters can tolerate an SWR of up to 3.0 for short transmissions, but anything higher than that risks burning out the transmitter.
Transmitting into a high SWR antenna will result in reduced signal strength, and will damage the transmitter rather quickly unless you have a transceiver with SWR protection. Since SWR protection just reduces the current to protect the output transistors, that results in a very low output power being radiated by a non-resonant antenna, so you won't be heard very well anyway.
If you have access to a VHF SWR meter, you could check the SWR for the broken antenna, and just continue using it if the SWR is less than 1:3.0, but most likely the SWR will be high.
You mentioned shortening your second antenna to match. You could check the coax from each antenna - most likely one antenna is connected to the flybridge radio and one to the lower helm radio. So you should have one working radio at the moment.
Anyway, this is just a long-winded way of saying that I recommend buying a new antenna.