There are a number of factors at play here. First of all, the mortgage even though it is joint really means that each of you are responsible for the whole amount. So your credit report is a mix of individual tradelines and joint tradelines. They are identified as such, but a joint is the same financial responsibility for debt to income calculations. Secondly, some of the recent regulations under the CARD act requires card issuers to confirm ability to pay at the time of underwriting. The ability to pay is required to be individual level not a household level. This was designed from the governments perspective to protect consumers from overextension of credit, and then being taken advantage of with high interest rates or additional fees. Of course this can affect households in two ways; one that a non working spouse is unable to obtain credit in their own name, and secondly, that individuals will not be able to take on as much new individual credit when they have tradelines that were underwriting with household income (a mortgage). If this sounds confusing, it is, and certainly falls into writing regulations that really have unintended consequences when put into practice. Each issuer can determine their own credit policy, but must be able to ensure government regulators that they have taken ability to pay into consideration.

Issuers can not ask for household income but they do try to find ways around that like asking for 'other income' to which you can answer what you feel comfortable is other income(household) and that should be used in the underwriting decision. Half the mortgage amout is not correct.

A long answer to your original question, but yes the new mortgage does have an effect on your ability to obtain additional credit. Plus if it is new, you have not built a payment history on this new and substantial debt. In the past, mortgage delinquencies were very rare, but not so now.