Tenacity can only be used on bermudagrass that is still dormant when trying to remove weeds or target grasses from it. If the bermuda is only semi dormant you will see whitening and damage to the bermuda. There are a few other potential options for you to remove the bentgrass from your bermuda lawn, though. Drive XLR8 is listed to control creeping bentgrass and has multiple rate options for it in bermuda lawns. You will need to use a Non Ionic Surfactant with the Drive XLR8 to acheive the best results. Revolver Herbicide is specifically labeled to target bentgrass as well in warm season turfs. Bonide Weed Beater Ultra is safe for use on bermuda and other turf species and specifically says to not use it on bentgrass. So while bentgrass is not listed as a target weed it makes it clear using it on bentgrass would be harmful to it. 

Revolver Herbicide is a post-emergent herbicide is a very good post emergent that is labeled to kill poa annua in Bermudagrass. Be sure to apply this product in temperatures above 60 degrees Fahrenheit. The mixing ratio is 0.2-0.4 fl. oz. per 1000 square feet. This will generally mix in a gallon of water.

You will need to use 0.4fl oz  - 0.8 fl oz per 1,000 sq ft of area that needs to be treated. Use the higher usage rate for hard to kill weeds. The amount of water used to apply Revolver herbicide is not critical, but should be sufficient for uniform coverage of the target area. Calibrate by determining the volume of water required to treat 1,000 square feet. Use this calibration volume to determine the amount of water and Revolver herbicide needed to treat the target area (see the following calibration example). Note: Sprayer calibration (volume of spray needed to treat 1,000 square feet) will vary with each individual operator.Steps in Calibration:1. Mark an area of 1,000 square feet (i.e. 20 by 50 feet or 25 by 40 feet).2. Place the sprayer on a level surface and add water noting the final level of water in the spray tank.3. Spray the marked area with a sufficient volume of water to provide uniform coverage. Refill the sprayer to the same level as before measuring the amount of water added. The measured water added to the sprayer is the volume needed to cover 1,000 square feet.4. Determine the application rate (0.4fl oz  - 0.8 fl oz per 1,000 sq ft.) for Revolver from the Recommendations section of this label.5. To each volume of water used as measured in step 3, add the amount of Revolver as determined in step 4.Example: If the sprayer uses 1 gallons of water to cover 1,000 square feet and the desired application rate of Revolver is .8 fl oz/1,000 square feet, then you would add .8  fluid ounces of Revolver to every 1 gallon of water to be used."