Fekete's lemma shows the existence of limits in subadditive sequences. This lemma, and generalisations of it, also have been used to prove the existence of thermodynamic limits in statistical mechanics. In this paper it is shown that the two variable supermultiplicative relation $$ p_{n_1}(m_1)\,p_{n_2}(m_2) \leq p_{n_1+n_2}(m_1 + m_2) $$ together with mild assumptions, imply the existence of the limit $$ \log \mathcal{P}_\#(ε) = \lim_{n\to\infty} \frac{1}{n} \log p_n(\lfloor εn \rfloor).$$ This is a generalisation of Fekete's lemma. The existence of $\mathcal{P}_\#(ε)$ are proven for models of adsorbing walks and polygons, and for pulled polygons. In addition, numerical data are presented estimating the general shape of $\log \mathcal{P}_\# (ε)$ of models of square lattice self-avoiding walks and polygons.