Any eight PARTS can have a Dual Insertion Effect. This includes the A/D Input PART. You select which eight PARTS as follows:
From SONG/PATTERN mode
Press [MIXING]
Press [F5] EFFECT>
Press [SF2] INS SWITCH

Here you can select any eight PARTs by turning its Part Switch ON
Only eight Parts can be checked, so it maybe necessary to turn a Part Switch Off, before you can select a different Part, only 8 red check marks can appear.

Press [F4] AUDIO IN
Press [SF1] OUTPUT

Here you can set the attributes for the A/D Input Part. Set the kind of input (mono/stereo), set its volume, route it to outputs (output select), sends to the System Reveb and Chorus Effects...

Press [SF2] CONNECT
Here you can choose the two Insertion Effects
Press [SF3]/[SF4] INS A and INS B
Here you can edit the two selected Insert Effects

In the case of the A/D Input, this is treated as one Part. The A/D Input is a single Input... It can be either a mono or a stereo input, but it is only one input. If you attempt to connect two mono devices, there is only a single preamp. That preamp controls both the Left and the Right channel. It is not two separate mono inputs, rather there is one A/D Input that can be mono or stereo.

Since this is an Audio Input, you can connect your guitar to the A/D Input and assign a Dual Insertion Effect, sample or record that Part with its two Effects to an Audio Track. Then in a separate pass, connect the Bass, assign a Dual Insertion Effect, sample or record that Part with its two Effects to an different Audio Track.

You cannot connect both the guitar and the bass simultaneously - if you did they would not only share the same preamp, the same volume, they would share the same Insert Effects, and the same output assignment.

This is because there is only one A/D Input designed to handle one mono input or one stereo input at a time. Hope that is clear.