Firstly, I do not understand your question. When you say you want the buttons to be working in "parallel" do you mean OR?
C1 is ON or X4 is OFF --> Energize Y1
C2 is ON or X4 is ON --> Energize Y2
Is X4 coming from a MOMENTARY PUSHBUTTON or from a MAINTAINED PUSHBUTTON (very unlikely) OR a SELECTOR SWITCH??? I think you meant SELECTOR SWITCH.
Is this what you are trying to do? If yes then you need to use a one-shot with your selector switch input and thus make it act like a momentary input. Then you need to use both the momentary inputs in parallel. However, this is not advisable since then the selector switch position might disagree with the actual mode of the process and this is very non-intuitive. If the operator does not / cannot look at the touch panel, he/she has no means of knowing what the actual mode is.
Also, another good programming practice in such cases is to use the SET and RST instructions in DirectSoft32. Use (SET) Y1 in parallel with (RST) Y2 for Manual mode and (RST) Y1 in parallel with (SET) Y2 for Auto mode. This way you dont have to worry about breaking the latches. One breaks when the other makes.
Now, if instead of parallel (OR), you meant series (AND) then it is actually easier. On rung 1 put XIC C1 in series with XIO X4 to SET Y1 and RST Y2 (in parallel). On rung 2 put XIC C2 in series with XIC X4 to SET Y2 and RST Y1 (in parallel again). Now, your selector switch (X4) acts as a permissive only. Based on the state of X4, C1 and C2 are allowed to change the mode respectively. And the state of X4 will never disagree with the actual mode. However, in this case, having two input sources (panel and switch) does not make much sense because one cannot function without the other.
Finally, if you want to make it a little smart, you can try this. Any of the two (panel or selector) can put the system in MANUAL but only the source which put the sytem in MANUAL can change it back to AUTO. This can be used to avoid safety hazards of two different people operating a system without knowledge of one another. I have tried this and can give you more details if you want. I think this is the best way to do it.
Summarizing,
If you want OR operation then both sources have independent control of the mode but the switch position can disagree with the actual mode -- safety concern.
If you want AND operation then the panel is dependent on the selector switch which only acts as a permissive to change the mode. However, saftey wise you are good.
Best option is to hand the control over to the one that operated it last. If MANUAL mode is safer than AUTO (usually it is) then the source that put system in MANUAL should be the only one to put it back it AUTO. This way, both can switch from AUTO to MANUAL but not vice versa.