So, you need to decide how many pounds they need to lose. The feed them the amount based on their ideal weight. Remember that treats, and chicken broth, have calories as well so you need to reduce the amount of kibble if you are going to do treats and chicken broth.
Since they weight different amoutns, the smaller one should always get less food. You do not have to be fair in this case.
I just looked at the Iams website. It does say on the bag how much to feed them if weight loss is deisred.
Try leaving the food out, with out chicken broth, for 24 hours and do not give treats. They will eat when they get hungry.
Sometimes the over the counter weight control diets are not strong enought to ellicit weight loss. So you keep feeding the correct amount and nothing happens. If this occurs, your vet can get you a prescription weight loss diet to feed them. They are so small that a bad of the prescription diet would last a while and not be all that expensive.
I am thinking you are not feeding them very much and they are still not losing weight. Time to go to a prescription diet.
Dogs can also have under active thyroid glands just like people. If you find they are not dropping weight as they should, a blood test can determine if they are hypo thyroid. If they are, they can take a supplement which will help increase their metabolism and help them lose weight.
Also, you need to increase their exercise for them to lose weight. Take them for more walks every day.
I hope this helps.