Nyquist noiseless channel capacity; how can bit-rate be two times the bandwidth?
I think you're confusing two different (but related) terms.

Nyquist says that in a channel of bandwidth $B$ you can transmit up to $2B$ orthogonal pulses per second. So, $R_p \leq 2B$, where $R_p$ is the pulse rate.

To achieve $R_p = 2B$, the pulses need to be sinc-shaped. Other, more practical pulses achieve slightly less than that. For example, raised cosine pulses with 50% excess bandwidth achieve $R_p = 1.5B$.

If you send pulses with $L=2^k$ amplitude levels, then the bit rate is $R_b = kR_p$.

So far for Nyquist, who was only interested in maintaining orthogonality. Shannon wanted to find a limit for the bit rate of reliable communication, with probability of error $P_b \rightarrow 0$. This limit is the capacity $C = B \log_2(1+\text{SNR})$, assuming an AWGN channel and orthogonal pulses. Note that $L$ is not mentioned explicitly.


  what is an example of a signal with capacity twice the bandwidth?


The capacity is a property of the channel, not the signal. A signal has a bandwidth, and implicitly a pulse rate and a bit rate. Regardless of that, the channel imposes a limit on the bit rate (its capacity), if you want reliable communication.