Twoport T-Network - Why does the voltage increase?
A current source has infinite internal resistance. Let's assume all resistors have the same value, 1 \$\Omega\$. Let's also assume that \$I_1=1\text{A}\$ and \$I_2\$ is either zero or equal to \$I_1\$.
a) \$I_2=0\text{A}\$ means \$R_B\$ will be in series with an infinite resitance, and both of them in parallel with \$R_C\$. The equivalent resistance will be \$R_e=1\Omega\$. The total resistance seen by \$I_1\$ will be \$R_A+R_e=2\Omega\$, thus the voltage will be \$U_1=2\Omega\cdot1\text{A}=2\text{V}\$. An alternative way of looking at this is to consider the current through \$R_B\$ as zero, and only \$1\text{A}\$ flowing through \$R_A\$ and \$R_C\$ from \$I_1\$.
b) \$I_2=1\text{A}\$ means the currents through \$R_A\$ and \$R_B\$ will be \$1\text{A}\$, for each, and for \$R_C\$ there will be \$2\text{A}\$ (one from each source). Thus the voltages across each resistors will be proportional to the currents, and each side will see a voltage of \$3\text{V}\$.