The answer to that is simple. It's the buy to let repayment lender that determines whether your investment is working for you.
Okay! okay! I understand each case is different and I know this may seem like a cliché, but it's the truth.
Every buy to let repayment lender that you'll find will have different rules and regulations in place. Their criteria will depend on what property you're buying, the rental income, and many other factors.
If you invest in multiple properties, it may be in your best interest to stick with one lender for everything. They will generally be more lenient in your overall debt ratio.
If you stick with them, their "stress test" (Calculation of maximum advance against each property rental income) will be more lenient. They may even consider the portfolio as a whole, instead of evaluating every property individually. This can help you tremendously in certain cases.
Sometimes, each different property that is financed would benefit from a different lender.
This may seem ridiculous but there are several reasons for this.
You may want to refurbish a property and put it back on the market for a quick profit.
The rental income from different properties could vary. You might benefit from separate "stress tests".
If you want to mix fixed and tracker rates, you may have to go to different lenders.
One lender may give traditionally low valuations of rental income.
You might let a family member live in the subject property. This can change things.
You may find a great buy to let lender, but they only allow a limited number of properties. If you want to expand, you'll have to go elsewhere.
One property could be in Scotland, the other in England
Some lenders don't want to lend for adjacent properties.
Keep in mind all of the above factors. It's never simple to find the correct combination of lenders and products to maxmise YOUR return from YOUR investments.