I'm testing a model with five latent constructs of which two are measured by only two indicators each. How do I take this into account?
David Kenny writes (http://davidakenny.net/cm/identify_formal.htm#B3b): "The construct has at least two indicators whose errors are uncorrelated and either a. both the indicators of the construct correlate with a third indicator of another construct but neither of the two indicators' errors is correlated with the error of that third indicator, or b. the two indicators' loadings are set equal to each other."
Can I check these error correlations in the Mplus output (e.g., RESIDUAL) or do I have to add something to the syntax in order to force the errors of the two indicatiors to be uncorrelated?
Your model is ok as is using the Mplus defaults. The residuals of the indicators are uncorrelated by default. Because you have other variables in the model than the two two-indicator sets the model is identified.