According to reports from various sources the managers at the Space Launch System are planning to utilize two mobile launchers (MLs). This will help in the additional use of the rocket for both crewed and cargo missions. 

Let’s take a keen look on the previous plans that were executed

The early mission of the SLS that is the block 1 version of the rocket was focussed for the test purpose only.  

The exploration mission- 1 and mission 2 are closely related to each other and an uncrewed Orion was sent for a test to the moon before the crewed one.

There are several refinements to this particular plan such as an evaluation to launch a crew on the first SLS mission. As the EM-1 plan was rejected, the team brought a more powerful, and impactful Block 1 B rocket, for better work.

 The Mobile Launcher-1 (ML-1)

The structure of the mobile launcher is a huge one modified from the Ares 1 role to cater to the Block 1 SLS. Such kinds of modification at least need two years to configure the ML for the tailor rocket. It is one of the primary reasons for the original EM-1 EM-2 missions to have a multiyear gap between them. 

While the MS is transported to the VAB there requires uninstalling many umbilical arms.  This helps in the installation of many shims before reinstalling the umbilical arms.

After all this, ML will be finally ready for EM-1 which is the original launch of the SLS on this mission. This mission in all probability will be taking place in 2020.

The Mobile Launcher-2 (ML-2)

The amount of investment is now confirmed to kick-start the process of constructing a new mobile launcher-2. This will be designed from the inception to be used only by the SLS block 1B variant. The investment is a little bit of this project but is definitely worth it.

Initially, the money will be paid to the team of civil engineers who will be designing the ML-2 to and build a basic structure of it.

Only the steel structure of ML- 2 may take more than 3 years, and then it may be require the support of different equipment which will also cost a huge sum of money. 

Apart from the SLS block 1 mission NASA also has planned for MSR flight.