Using a PNP transistor as a switch connected from a 74154 then out to stepper motors
Prinzipal your selection with 74154 is a bad chip, because you can never switch off all outputs and more bad this solution makes you not free to use every Stepper separate.
Better you use the SPI serial bus of the MCU and connect it to 2x 74HC595.
Reset the #OE Pin 13

Connect follow Pins to the chips:

1) MOSI from MCU to SER(pin14, first 74HC595) 

2) QH from first '595 to SER second '595

3) SPI_CLK from MCU to both '595 at pins SRCLK(11)

4) SPI_CS from MCU to both '595 at pin RCLK(12)

5) #SRCLR pins connect over 47k resistor to VCC

6) Than you can connect all outputs to 4x Resistor array 4x47k to VCC

7) connect all outputs to ULN-Chips.