Can lentiSAMv2 (with a guide RNA) activate without using lentiMPH v2? I recall that lentiMPH v2 just improves the efficiency of activation for some genes.
It really depends on your gene and cell type and the level of activation you need to get your phenotype. Using lentiSAMv2 with lentiMPHv2 is a safer bet because there are a lot of cases where using only lentiSAMv2 (especially in a protein-limited, lentiviral setting) will not significantly activate your gene of interest. I would recommend experimentally testing this out for your particular gene and cell type.