Mass spectrometry (MS), particularly targeted proteomics, is increasingly being used for quantifying specific proteins and peptides in clinical specimens. The coupling of immuno-enrichment of proteotypic peptides with MS [e.g., immuno-multiple reaction monitoring (MRM) and immuno-matrix-assisted laser desorption ionization (MALDI)] enables the development of highly sensitive and specific assays for low-abundance signaling proteins. By incorporating stable isotope-labeled standards, these workflows allow the determination of endogenous protein concentrations. This is typically achieved through external calibration, often using surrogate matrices, which has inherent limitations for the analysis of clinical specimens as there are often substantial variations in the sample matrix, and sample amounts are typically limited. We have previously introduced the use of two peptide isotopologues for generating external calibration curves in plasma. Here, we present a two-point internal calibration (2-PIC) strategy using two isotopologues for immuno-MS assays and demonstrate its flexibility and robustness. Quantification of the tumor suppressor PTEN in Colo-205 cells by immuno-MRM and immuno-MALDI using 2-PIC and external calibration yielded very similar results (relative standard deviation between 2-PIC and external calibration: 4.9% for immuno-MRM; 1.1% for immuno-MALDI), without the need for a surrogate matrix or additional patient material for calibration, while concurrently reducing the instrument time and cost. Although our PTEN immuno-MRM and immuno-MALDI assays can be considered to be orthogonal as they utilized entirely different sample preparation and MS analysis workflows, targeted different PTEN peptides, and were performed in different laboratories, the endogenous Colo-205 PTEN levels determined with 2-PIC showed a good correlation (r2 = 0.9966) and good agreement (0.48 ± 0.01 and 0.29 ± 0.02 fmol/μg of total protein) between immuno-MRM and immuno-MALDI.