If you are interested in the benefits of a second mortgage, you should use a second mortgage calculator to see if the second mortgage rate charged is reasonable and if it will indeed save you money in the long run.
Before considering taking out a second mortgage, you should clarify the following:

When can the company adjust the interest on the second loan rate?

Are there any limits to the amount of interest on the second loan rate or payments charged?

How many times and at what time intervals can the company change the second mortgage rate interest?

How will the company determine the new rate of interest or the second loan rate?
Also, another point to consider is that second loan rates are generally higher than the first mortgage rates and the terms of the second mortgage are shorter, due to the risk of the loan. Since property prices have skyrocketed most people are going in for second mortgages with lower second mortgage rates and second loan rates.