Unkown connection number for ModelingToolkit system

Specific Domains Modelling & Simulations
4

Do you mean something like this?

using ModelingToolkit

@variables t
D = Differential(t)

@named inner_sys = ODESystem(Equation[], t)

function subsystem(;name)
    states = @variables X(t)=0. In(t)=0.
    eqs = [X ~ In]

    ODESystem(eqs, t, states, []; name=name)
end

@named sub = subsystem()
@named sys = compose(inner_sys, sub)

function input_factory(;name)
    states = @variables Y(t)
    eq = [
        D(Y) ~ -Y
    ]
    ODESystem(eq, t, states, []; name=name)
end

@named inputs[1:2] = input_factory();
@named tmp = ODESystem([sub.In ~ sum(input->input.Y, inputs)])
@named sys_full = compose(extend(sys, tmp), inputs)

it gives

julia> equations(sys_full)
4-element Vector{Equation}:
 sub₊In(t) ~ inputs_1₊Y(t) + inputs_2₊Y(t)
 sub₊X(t) ~ sub₊In(t)
 Differential(t)(inputs_1₊Y(t)) ~ -inputs_1₊Y(t)
 Differential(t)(inputs_2₊Y(t)) ~ -inputs_2₊Y(t)